Tissue diagnosis is important in many fields of medicine, including, but not limited to: gastrointestinal, cardiovascular, urological, pulmonary, reproductive, dermatology, surgery, and general medicine. For example, early detection of tissue malignancy is essential to avoid the spread of cancer and associated complications. In the gastrointestinal tract, for example, endoscopic and/or laparoscopic "minimally invasive" techniques can be used to obtain a biopsy that provides a physical sample of a tissue site. The tissue site can be either a flat surface or subsurface mucosal lesion or a raised mucosal lesion (e.g., a polyp). The biopsy can be analyzed in a pathology laboratory using histopathological techniques to determine whether it is cancerous. The tissue may be normal, hyperplastic, adenomatous, or malignant. For example, hyperplastic polyps consist of normal tissue, and are therefore benign. Adenomatous polyps, which are also referred to as dysplastic polyps, consist of abnormal tissue, and are a risk of future malignancy. Adenocarcinomas are malignant polyps that pose an immediate risk of spreading to other areas of the body.
Histopathology, while relatively accurate, requires the physical removal of a tissue sample and its time-consuming analysis in a pathology laboratory. Further treatment of the tissue site based on the results of the histopathological analysis may require a second medical procedure, separate from the original diagnostic procedure that obtained the biopsy. Along with an increased cost and patient discomfort, locating the original biopsy site may be extremely difficult. Moreover, gathering a physical biopsy sample is not without risk, since it typically involves cutting and removing a small portion of tissue. For this reason, taking unnecessary physical biopsy samples should be avoided. Sampled tissue sites that are subsequently found to be hyperplastic by histopathological analysis were, in retrospect, unnecessarily sampled.
Moreover, some patients undergoing endoscopic colonic examination, for example, will have an abundance of small (e.g., less than 5 millimeters in diameter) polyps that are either hyperplastic or adenomatous. Since sampling each site is difficult and increases the risk of other complications, physical samples are obtained from only a "representative" subpopulation of the sites. However, this leaves other possibly premalignant sites undiagnosed, even though such sites could become malignant and contribute to the spread of cancer in the patient. Thus, the risk of obtaining physical biopsy samples is compounded when only a subpopulation of the sites is sampled.
In deciding whether to remove a physical biopsy sample for histopathological analysis, an endoscopist typically subjectively determines visually whether a polyp is hyperplastic or adenomatous. The accuracy of existing biopsy methods depends upon the endoscopist's ability to subjectively determine healthy from suspicious tissue to biopsy. However, it is difficult, if not impossible, to visually differentiate between small hyperplastic and small adenomatous polyps, particularly when viewed through the viewing optics of an endoscope. Moreover, because conditions other than cancer can cause tissue discoloration, an accurate visual characterization is extremely difficult, and histopathological analysis of a physical tissue sample is often required. As a result, the subjective visual inspection may leave adenomatous polyps undiagnosed and therefore untreated.
Various tissue classification techniques have also been developed as alternatives or adjuncts to physical biopsy sampling and visual differentiation between tissue characteristics. One class of such techniques involves illuminating tissue with incident light, and allowing the incident light energy to interact with the tissue. The tissue is classified based on light that is returned from the tissue. A particularly interesting class of such techniques, referred to as fluorescence spectroscopy, is based on the observation that different tissue characteristics result in a different fluorescence in the returned light. More particularly, spectral characteristics of the fluorescence returned from premalignant or malignant tissue may be different from that returned from normal or benign tissue.
Many such fluorescence-based techniques depend on the use of extrinsic fluorescence-enhancing dyes, stains, or other image contrast agents. Contrast agents are typically substances that are ingested by the patient, delivered intravenously, or delivered locally to a tissue site to enhance its fluorescence. A contrast agent is known to substantially target only the particular type of tissue being detected, and to increase the fluorescence properties of that type of tissue for obtaining a better image. Contrast agents pose at least two problems. First, their selectivity is less than optimal. Tissue uptake and concentration levels may be significantly variable. The contrast agent attaches to other types of tissue as well as the targeted tissue. This hinders an accurate diagnosis based on observation of returned fluorescence. Second, certain contrast agents have undesirable side-effects, such as acute and/or chronic light-sensitivity of the patient. Thus, fluorescence techniques using extrinsic fluorescence-enhancing agents for diagnosis have limited usefulness.
Other techniques avoid the use of extrinsic fluorescence-enhancing agents, depending instead on native fluorescence (also referred to as autofluorescence) from endogenous tissue. Even without contrast agents, the spectral characteristics of the fluorescence returned from premalignant or malignant tissue may be different from that returned from normal or benign tissue. Such differences, however, are much less pronounced in the absence of extrinsic image contrast agents. Detecting small differences between spectral fluorescence characteristics of different tissue types is much more difficult without using extrinsic image contrast agents. As a result, such systems require complicated and expensive components, such as multiple optical fibers for illuminating or collecting returned fluorescence from the tissue, or image intensification or photomultiplication devices for obtaining an adequate signal from the returned fluorescence.
Other systems do not provide the physician with an actual diagnosis based on tissue classification using fluorescence data. For example, Palcic et al., U.S. Pat. No. 5,507,287 entitled "ENDOSCOPIC IMAGING SYSTEM FOR DISEASED TISSUE," produces a pseudo-color image of the tissue based on the returned fluorescence from the tissue. However, the attending physician must still try to subjectively diagnose the tissue based on the pseudo-image provided on the display.
Even if tissue is accurately diagnosed, by the physician, or otherwise, using endoscopic techniques, treating tissue diagnosed as abnormal is still difficult. Many systems require an exchange of diagnostic and treatment devices. For example, in systems using multiple optical fibers extending through the working channel of an endoscope to diagnose the tissue, the diagnosing optics are removed from the working channel of an endoscope so that a forceps, snare, or ablation device can be extended through the working channel of an endoscope to treat the tissue. However, exchanging diagnostic and treatment devices poses problems. In the colon, for example, inherent colonic motility makes it difficult for the physician to accurately maintain the position of the endoscope during the exchange of diagnosing and treatment devices. As a result, the physician may not be able to locate the previously diagnosed polyp or may inadvertently treat the wrong polyp. Thus, exchanging diagnosing and treatment devices reduces the efficacy of the medical procedure.
In summary, there is a critical medical need for accurate and early diagnosis and treatment of premalignant and malignant tissue to prevent the spread of cancer. Risks and other disadvantages with obtaining physical biopsy samples for histopathological analysis indicate a need for improved techniques for classifying tissue. There is a need for providing accurate diagnosis and immediate treatment of premalignant and malignant tissue, without requiring multiple medical procedures, without using extrinsic agents for enhancing fluorescence, and without using complicated and expensive components in the absence of such contrast agents.